Fix time widget to display 00:00

time(0, 0) is evaluated to False so the test for None must be explicit
to set correctly 00:00.

issue4969
review22501002

tryton/common/datetime_.py

@@ -275,7 +275,7 @@ class Time(gtk.ComboBoxEntry):
         self.__time = time
 
     def update_label(self):
-        if not self.__time:
+        if self.__time is None:
             self.__entry.set_text('')
             return